Helper script: implement prebuilt tarball handling

Occasionally we may want to use prebuilt, officially released Qt5 module
tarballs instead of generating them from git. This change adds the
possibility to specify the path where official tarballs are located,
overriding the usual mode of operation.

When using prebuilt tarballs, the helper script requires to specify
which modules are to be uploaded.

To ease future testing, there is now a new option '--skip-obs-commit'.
Helper: fix generated githash revision name

The passed argument already contains the "g$HASH" string, so there's no
need to add another prefix.
Merge branch 'tarballs'
QtQuick1: update packaging

- changed qmake invocation; using %qmake macro in specfile broke the
  build, plain qmake call works as it should
- new library names (QtQuick1 -> Qt5Declarative)
- new import paths